Boiler control comprising analog up/down timer circuit for generating variable threshold signal

ABSTRACT

An apparatus comprises a sensor circuit configured to detect activation of at least one circulator arranged to circulate liquid from a boiler through at least one circulation loop and back to the boiler. An analog up/down timer circuit has an input coupled to an output of the sensor circuit and generates a variable threshold signal that varies as a function of an activation time of the at least one circulator. A burner control circuit receives the variable threshold signal from the analog up/down timer circuit and generates an ignition control signal based at least in part on comparison of a temperature sensor signal of the boiler with the variable threshold signal. An ignition driver receives the ignition control signal from the burner control circuit and generates an ignition signal for a burner configured to burn fuel to heat the liquid in the boiler based at least in part on the ignition control signal.

RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.15/017,054 filed on Feb. 5, 2016, entitled “Boiler Control ComprisingAnalog Up/Down Timer Circuit for Generating Variable Threshold Signal,”which is a continuation-in-part of U.S. patent application Ser. No.14/613,505, filed Feb. 4, 2015, entitled “Analog Timer Circuit with TimeConstant Multiplication Effect,” and now issued as U.S. Pat. No.9,287,770, which claims the priority of U.S. Provisional PatentApplication Ser. No. 62/386,930, filed Dec. 16, 2015, and entitled “HighReliability Energy Saving Hot Water Boiler Control,” the disclosures ofwhich are incorporated by reference herein.

FIELD

The field relates generally to electronic circuitry, and moreparticularly to electronic control circuitry.

BACKGROUND

A wide variety of different types of electronic control circuitry areknown. These include, for example, boiler controls for hot water heatingsystems. A typical hot water boiler control utilizes a switch responsiveto an output signal from a boiler water temperature sensor to controlburner fuel supply and ignition. The boiler water temperature sensor maycomprise a heat sensitive capillary tube inserted into an enclosed wellin the hot water boiler. The hot water heating system further comprisesa room or area thermostat that is used to control a circulator. Thecirculator is configured to circulate hot water from the boiler throughhot water radiators associated with the room or area in order tomaintain a desired room or area temperature.

In these and other typical boiler controls, temperature settings of theswitch that controls burner fuel supply and ignition are confined to arelatively narrow range selected to accommodate the coldest outsidetemperature that is likely to be experienced. More particularly, thetemperature settings generally must be high enough for the coldestoutdoor conditions likely to be experienced so that the hot waterradiators will be capable of providing a comfortable room or areatemperature under those conditions. However, at higher boilertemperatures there is greater heat loss, greater fuel usage and greatergreenhouse emissions. Since outside temperatures vary throughout theyear, it is advantageous and more efficient for boiler temperatures tobe reduced as outside temperatures increase. Comfortable room or areatemperatures can be maintained using significantly reduced boilertemperatures in the presence of higher outdoor temperatures. This isespecially important during spring, summer and fall times when theboiler is not used for heating but merely to provide hot water via itsinternal hot water heat exchangers. Under these conditions, high boilertemperatures are not needed, are particularly inefficient andunnecessarily produce excessive greenhouse emissions.

A number of different approaches attempt to address the inefficienciesassociated with maintaining unduly high boiler temperatures. Forexample, some of these approaches involve measuring the outsidetemperature and reducing boiler temperature as the outside temperatureincreases. Other approaches involve comparing the measured temperatureof the water leaving the boiler and going to the hot water radiators tothe measured temperature of the water returning to the boiler from thehot water radiators, and reducing boiler temperature as the differencebetween the measured temperatures decreases.

Although these techniques can be effective, they suffer from a number ofsignificant drawbacks. For example, some techniques require the additionof outdoor temperature sensors or the utilization of complex digitaldata processors and associated software. This adds excessive cost to theboiler control and to its installation. This also increases the numberof boiler control failure modes. For example, single point failures ofcritical components can cause a temperature runaway condition in theboiler.

SUMMARY

Illustrative embodiments of the present invention provide electroniccontrol circuitry particularly well suited for use with a hot waterboiler. For example, some embodiments provide boiler control circuitryin which boiler temperature is automatically reduced as outsidetemperature increases without the need for outdoor temperature sensorsor complex digital data processor controls. Furthermore, someembodiments incorporate dual boiler temperature sensing, fault detectionand associated multiple burner ignition relays in order to greatlyenhance reliability and prevent boiler temperature runaway conditions.

These and other embodiments advantageously provide a boiler control thatexhibits exceptionally high reliability at low cost and without theother drawbacks of the conventional arrangements described previously.Such a boiler control can significantly reduce fuel consumption andassociated greenhouse emissions for both home and commercial heatingsystems, and in other applications.

In one embodiment, an apparatus comprises a sensor circuit, an analogup/down timer circuit, a burner control circuit and an ignition driver.The sensor circuit is configured to detect activation of at least onecirculator arranged to circulate liquid from a boiler through at leastone circulation loop and back to the boiler. For example, there may be aplurality of circulators and the sensor circuit may illustrativelycomprise a current sense circuit arranged in series with respectivealternating-current control lines of the plurality of circulators.

The analog up/down timer circuit in this particular embodiment has aninput coupled to an output of the sensor circuit and is configured togenerate a variable threshold signal that varies as a function of anactivation time of said at least one circulator. The burner controlcircuit is configured to receive the variable threshold signal from theanalog up/down timer circuit and to generate an ignition control signalbased at least in part on comparison of a temperature sensor signal ofthe boiler with the variable threshold signal. The ignition driver isconfigured to receive the ignition control signal from the burnercontrol circuit and to generate an ignition signal for a burnerconfigured to burn fuel to heat the liquid in the boiler based at leastin part on the ignition control signal.

By way of example, the analog up/down timer circuit may illustrativelycomprise a pulse source, a charge storage element, a charge pump coupledbetween the pulse source and the charge storage element, and a dischargepump coupled between the pulse source and the charge storage element. Insuch an arrangement, a pulse signal generated by the pulse source isillustratively utilized to charge the charge storage element via thecharge pump in a charge mode of operation for a period of time for whichat least one of a plurality of circulators is activated and to dischargethe charge storage element via the discharge pump in a discharge mode ofoperation for a period of time for which none of the plurality ofcirculators is activated. The analog up/down timer circuit may beconfigured in a particular one of the charge and discharge modes ofoperation responsive to an up/down control signal generated by thesensor circuit. Additionally, the charge pump and the discharge pump maybe driven by respective ones of complemented and uncomplemented versionsof the pulse signal generated by the pulse source.

In some embodiments, a boiler control comprises a dual boiler sensecircuit, with the dual boiler sense circuit comprising a firsttemperature sensor configured to generate the temperature sensor signalas a first temperature sensor voltage, and a second temperature sensorconfigured to generate a second temperature sensor voltage. Suchembodiments may further comprise a fault detection circuit configured tomonitor the first and second temperature sensor voltages and to generatea fault indication signal if the first and second temperature sensorvoltages differ from one another by more than a designated amount.

In some embodiments, the ignition driver illustratively comprises afirst relay circuit configured to control a state of a first switcharranged in series with an alternating-current control line of theburner based at least in part on the ignition signal. Such embodimentsmay further comprise a fault driver, with the fault driver comprising asecond relay circuit configured to control a state of a second switcharranged in series with the first switch and the alternating-currentcontrol line of the burner based at least in part on a fault indicationsignal generated by a fault detection circuit.

Other embodiments include, by way of example and without limitations,boiler control methods as well as heating systems that comprise boilercontrol circuitry.

Electronic control circuitry in other embodiments can be implemented ina wide variety of other control system applications. Such electroniccontrol circuitry may but need not comprise an analog timer circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an analog timer circuit in an illustrativeembodiment of the invention.

FIG. 2 is a schematic diagram showing one possible implementation of theanalog timer circuit of FIG. 1.

FIGS. 3 and 4 show respective illustrative embodiments of feedbackcontrol systems incorporating an analog timer circuit of the typedescribed in conjunction with FIGS. 1 and 2.

FIG. 5 is a block diagram of boiler control circuitry comprising ananalog timer circuit in an illustrative embodiment.

FIGS. 6 and 7 are schematic diagrams showing possible implementations ofportions of the boiler control circuitry of FIG. 5.

FIG. 8 is a block diagram of boiler control circuitry comprising anauxiliary heater in an illustrative embodiment.

FIGS. 9, 10 and 11 are schematic diagrams of respective relay-basedimplementations of portions of the boiler control circuitry of FIG. 8.

FIGS. 12, 13 and 14 are schematic diagrams of respective solid statebased implementations of portions of the boiler control circuitry ofFIG. 8.

DETAILED DESCRIPTION

Illustrative embodiments of the present invention will be describedherein with reference to examples of analog timer circuits and othertypes of electronic control circuitry, as well as systems thatincorporate such electronic control circuitry. It is to be appreciated,however, that embodiments of the invention are not restricted to usewith the particular illustrative circuit and system configurationsshown. For example, those skilled in the art will recognize thatnumerous alternative electronic control circuitry configurations can beimplemented utilizing the teachings provided herein. Moreover,embodiments of the invention are not limited to the particular controlsystems shown, but are more generally applicable to any type of systemin which it is desirable to provide enhanced control functionality.Accordingly, the term “system” as used herein is intended to be broadlyconstrued so as to encompass a wide variety of different devices orarrangements of multiple devices of the type disclosed herein.

Some embodiments of the invention utilize analog timer circuits of thetype disclosed in the above-cited U.S. patent application Ser. No.14/613,505. For example, hot water boiler control circuitry thatincorporates one possible implementation of an analog timer circuit willbe described below in conjunction with FIGS. 5, 6 and 7. It is to beappreciated, however, that other hot water boiler control circuitryembodiments of the invention need not utilize analog timer circuits.Examples of such embodiments will be described in detail below inconjunction with FIGS. 9 through 14.

Before describing the hot water boiler control circuitry embodiments,example analog timer circuit embodiments will be described withreference to FIGS. 1 through 4.

It should initially be noted that a wide variety of electronic timercircuits are known in the art, including both analog timer circuits anddigital timer circuits. Conventional analog timer circuits based onresistor-capacitor (RC) time constants are advantageous in that suchcircuits can be configured to provide a substantially continuous analogoutput and thus a large number of possible set points. However, suchanalog timer circuits can be problematic in applications in whichrelatively long time constants, on the order of tens, hundreds orthousands of seconds, are needed, as large and costly capacitors can berequired.

Digital timer circuits based on digital counters overcome some of thedisadvantages of analog timer circuits. For example, digital timercircuits can provide relatively long time constants. However, suchdigital timer circuits exhibit only a limited number of set points,corresponding to respective predetermined discrete output times, ascompared to the much larger number of set points provided by thecontinuous analog output of an analog timer circuit. Moreover, digitaltimer circuits are generally more complex than analog timer circuits,and some of the digital circuit elements can be very costly.

Various techniques are also known for increasing the effectivecapacitance value of a given capacitive element. For example, thewell-known Miller effect provides an increase in the effectivecapacitance of a given capacitor when the capacitor is used as anegative feedback element in a grounded-emitter bipolar transistoramplifier. In this configuration, the increase in effective capacitanceapproaches the current gain or beta of the transistor. A similarincrease in effective capacitance can be achieved in an emitter-followercircuit in which the capacitor is connected between the base andcollector of the bipolar transistor. As the maximum beta value for abipolar transistor is typically about 100, the increase in effectivecapacitance in these and other similar conventional arrangements is atmost about 100:1, which is generally insufficient to overcome theabove-noted issues arising in conventional analog timer circuits basedon RC time constants.

Illustrative embodiments of the present invention provide simple andinexpensive analog timer circuits that exhibit an advantageous timeconstant multiplication effect. For example, a given embodiment canprovide a long time constant, on the order of tens, hundreds orthousands of seconds, using a much smaller capacitance value than wouldbe required in a conventional analog timer circuit based on acorresponding RC time constant. More particularly, in some of theillustrative embodiments, a time constant multiplication factor of up to5000:1 or more is provided, such that an analog timer circuit providingrelatively long time constants can be implemented using inexpensivecomponents. Circuit reliability is also enhanced by the application ofrelatively low impedance circuit elements while achieving long timeconstants.

In one embodiment, an analog timer circuit comprises a pulse source, acharge storage element, and a charge pump coupled between the pulsesource and the charge storage element. A pulse signal generated by thepulse source is utilized to charge the charge storage element via thecharge pump.

The pulse signal may comprise an asymmetric pulse signal, with a chargetime constant of the analog timer circuit being controllable throughadjustment of at least one of a frequency and an asymmetry of the pulsesignal.

The charge pump and charge storage element may comprise respectivecapacitive elements, with the charge time constant of the analog timercircuit being controllable through adjustment of relative capacitancevalues of the respective capacitive elements of the charge pump and thecharge storage element.

The analog timer circuit may further comprise a charge distributioncircuit coupled between the charge pump and the charge storage element.The charge distribution circuit comprises first and second circuit pathsseparating energy from the charge pump into respective first and secondportions, with the first circuit path providing the first portion of thecharge pump energy to the charge storage element, and the second circuitpath diverting the second portion of the charge pump energy away fromthe charge storage element. In such an arrangement, the charge timeconstant of the analog timer circuit is controllable through adjustmentof relative distribution of the charge pump energy between the first andsecond circuit paths.

The analog timer circuit may further comprise a discharge pump coupledbetween the pulse source and the charge storage element, with the pulsesignal generated by the pulse source also being utilized to dischargethe charge storage element via the discharge pump. For example, thecharge pump and the discharge pump may be driven by respective ones ofcomplemented and uncomplemented versions of the pulse signal generatedby the pulse source.

Time constant adjustment mechanisms similar to those described above forthe charge time constant may be provided in an analogous manner for adischarge time constant of the analog timer circuit.

An up/down driver circuit may be provided to select between charging ofthe charge storage element via the charge pump and discharging of thecharge storage element via the discharge pump.

Other embodiments include, by way of example and without limitation,systems that implement feedback control utilizing an analog timercircuit.

Illustrative embodiments of analog timer circuits as disclosed hereincan provide significant advantages relative to conventionalarrangements. For example, relatively long time constants can beprovided using low capacitance and resistance values, and thus withsignificantly less cost and complexity than would otherwise be required.Also, an analog timer circuit in a given embodiment can be configured toallow for thousands of set points as compared to the limited number ofoutputs from a digital timer circuit.

FIG. 1 shows a block diagram of an analog timer circuit 100 in oneembodiment. The analog timer circuit 100 comprises an astablemultivibrator 102 that is coupled via an inverter 104 to a charge pump106. The analog timer circuit 100 further comprises a chargedistribution circuit 108, a charge storage element 110, an up/downdriver circuit 112, an output driver 114, a discharge pump 116 and adischarge distribution circuit 118.

The analog timer circuit 100 is advantageously configured to allow agiven capacitive element comprising the charge storage element 110 toperform as if it were a much larger capacitive element. Accordingly,much longer time constants can be provided than would otherwise bepossible using the given capacitive element. For example, it is possiblein some embodiments to provide as much as a 5000 times increase or morein time constant length over that provided using a correspondingequivalent RC time constant arrangement. Embodiments of the inventiontherefore extend the time constant achievable using a given capacitiveelement by multiple orders of magnitude relative to conventional analogtimer circuits based on RC time constants.

The analog timer circuit 100 is configured as an up/down timer circuit,in that it can selectively provide timing functionality based oncharging of the charge storage element 110 and discharging of the chargestorage element 110 in respective charge and discharge modes ofoperation of the analog timer circuit 100.

The up/down driver circuit 112 is configured to select between chargingof the charge storage element 110 via the charge pump 106 anddischarging of the charge storage element 110 via the discharge pump116, responsive to an up/down select signal. More particularly, theup/down driver circuit 112 prevents the discharge pump energy from beingdelivered by the discharge distribution circuit 118 to the chargestorage element 110 in the charge mode of operation, and prevents thecharge pump energy from being delivered by the charge distributioncircuit 108 to the charge storage element 110 in the discharge mode ofoperation, thereby controlling whether the charge on the charge storageelement 110 will increase or decrease.

The output driver 114 is configured as an isolation driver whichpresents a high input impedance to the charge storage element 110 toprevent loading of the corresponding capacitive element. The outputdriver 114 also provides a low direct current (DC) output impedance forthe analog timer circuit 100.

The output driver 114 in the present embodiment provides a substantiallycontinuous analog output that can be slowly increasing or slowlydecreasing, depending upon the selected charge or discharge mode ofoperation, and in accordance with the configured charge or dischargetime constant.

By way of example, the output of the analog timer circuit 100 can beused in applications in which a control event is to be initiated after arelatively long delay. The output of the analog timer circuit can becompared to a preselected DC level and after the long delay thedesignated control event is initiated.

It is to be appreciated, however, that other types of analog timercircuit outputs can be provided in other embodiments.

A pulse signal generated by the astable multivibrator 102 is utilized tocharge the charge storage element 110 via the charge pump 106, and todischarge the charge storage element 110 via the discharge pump 116. Theastable multivibrator 102 is an example of what is more generallyreferred to herein as a “pulse source.” Numerous other types of pulsesources may be used in other embodiments, such as, for example, a pulsegenerator.

The charge pump 106 and the discharge pump 116 in some embodiments areimplemented as respective voltage source electrical energy pumps eachcomprising a single capacitive element, although numerous other types ofcharge and discharge pumps can be used in other embodiments. Charge anddischarge pumps implemented using respective single capacitive elementsare also referred to herein as capacitor charge pumps and capacitordischarge pumps, respectively.

Possible alternatives to the capacitor charge and discharge pumpsutilized in certain embodiments include, for example, inductive chargeor discharge pumps, in which energy from pulses of a pulse signal isinductively delivered to or removed from the charge storage element 110.Other arrangements involving provision of pulse energy to or from thecharge storage element 110 can be used.

The pulse signal generated by the multivibrator 102 illustrativelycomprises an asymmetric pulse signal. More particularly, the pulsesignal is assumed to comprise an asymmetric square wave. Other types ofpulse signals can be used in other embodiments.

In the present embodiment, a charge time constant of the analog timercircuit 100 is controllable through adjustment of at least one of afrequency and an asymmetry of the pulse signal generated by themultivibrator 102.

The inverter 104 has its input coupled to an output of the multivibrator102. The inverter 104 receives at its input an uncomplemented version ofthe pulse signal generated by the multivibrator 102 and provides at itsoutput a complemented version of the pulse signal. Accordingly, theinput of the charge pump 106 in this embodiment is driven by thecomplemented version of the pulse signal.

The charge pump 106 and the charge storage element 110 illustrativelycomprise respective capacitive elements.

The above-noted charge time constant of the analog timer circuit 100 isfurther controllable through adjustment of relative capacitance valuesof the respective capacitive elements of the charge pump 106 and thecharge storage element 110.

Additional control of the charge time constant of the analog timercircuit 100 is provided by adjustment of the charge distribution circuit108. For example, the charge distribution circuit 108 may comprise firstand second circuit paths separating energy from the charge pump 106 intorespective first and second portions, with the first circuit pathproviding the first portion of the charge pump energy to the chargestorage element 110, and the second circuit path diverting the secondportion of the charge pump energy away from the charge storage element110. In one implementation of such an arrangement, the chargedistribution circuit 108 illustratively dissipates part of the energyreceived from the charge pump 106 and feeds the remaining receivedcharge pump energy to the charge storage element 110.

The charge time constant of the analog timer circuit 100 is thencontrollable through adjustment of relative distribution of the chargepump energy between the first and second circuit paths.

Accordingly, the charge time constant in the analog timer circuit 100 iscontrollable through adjustment of one or more of the following:

1. Frequency of the pulse signal generated by the multivibrator 102.

2. Asymmetry of the pulse signal generator by the multivibrator 102.

3. Relative capacitance values of the respective capacitive elements ofthe charge pump 106 and the charge storage element 110.

4. Relative distribution of the charge pump energy between the first andsecond circuit paths of the charge distribution circuit 108.

It is to be appreciated that a given embodiment may provide only asubset of the adjustability mechanisms described above, and possiblyadditional or alternative adjustment mechanisms.

A discharge time constant of the analog timer circuit 100 iscontrollable in a manner similar to that described above for the chargetime constant.

More particularly, in this embodiment the charge pump 106 and thedischarge pump 116 are driven by respective complemented anduncomplemented versions of the pulse signal generated by themultivibrator 102.

As a result, the discharge time constant of the analog timer circuit 100is also controllable through adjustment of at least one of a frequencyand an asymmetry of the pulse signal generated by the multivibrator 102.

It should be noted that the inverter 104 can instead be arranged betweenthe multivibrator 102 and the discharge pump 116, rather than betweenthe multivibrator 102 and the charge pump 106 as in the FIG. 1embodiment, such that the charge pump and discharge pump are driven byrespective uncomplemented and complemented versions of the pulse signalgenerated by the multivibrator 102.

Like the charge pump 106, the discharge pump 116 may comprise acapacitive element. The discharge time constant of the analog timercircuit 100 is then controllable through adjustment of relativecapacitance values of the respective capacitive elements of thedischarge pump 116 and the charge storage element 110.

Like the charge distribution circuit 108, the discharge distributioncircuit 118 illustratively comprises first and second circuit pathsseparating energy from the discharge pump 116 into respective first andsecond portions, with the first circuit path providing the first portionof the discharge pump energy to the charge storage element 110, and thesecond circuit path diverting the second portion of the discharge pumpenergy away from the charge storage element 110. In one implementationof such an arrangement, the discharge distribution circuit 118illustratively dissipates part of the energy received from the dischargepump 116 and feeds the remaining received discharge pump energy to thecharge storage element 110.

The discharge time constant of the analog timer circuit 100 is thencontrollable through adjustment of relative distribution of thedischarge pump energy between the first and second circuit paths.

Again, other embodiments can utilize only a subset of the particularadjustment mechanisms described above for controlling the discharge timeconstant of the analog timer circuit 100.

It should also be understood that the particular set of componentsimplemented in the analog timer circuit 100 as illustrated in FIG. 1 arepresented by way of example only. In other embodiments, only subsets ofthese components, or additional or alternative sets of components, maybe used, and such components may exhibit alternative functionality andconfigurations.

For example, although configured in the FIG. 1 embodiment as an up/downtimer, the analog timer circuit 100 in other embodiments can bereconfigured as an up-only timer by eliminating the up/down drivercircuit 112, the discharge pump 116 and the discharge distributioncircuit 118, and providing a reset mechanism for discharging the chargestorage element 110 after a charging threshold is reached. Similarly,the analog timer circuit 100 in other embodiments can be reconfigured asa down-only timer by eliminating the up/down driver circuit 112, thecharge pump 106 and the charge distribution circuit 108, and providing areset mechanism for charging the charge storage element 110 after adischarging threshold is reached.

It should be noted in this regard that the term “analog timer circuit”as used herein is intended to be broadly construed, so as to encompass,for example, any timer circuit arrangement in which the primary timingmechanism is based on a substantially continuous analog signal.Accordingly, it is possible that a given analog timer circuit asdisclosed herein may include a certain minimal number of digitalcomponents, such as, for example, digital switches for controllingswitching between charge and discharge modes of operation, but such ananalog timer circuit does not utilize digital timing circuitry such asdigital counters for its primary timing mechanism.

Referring now to FIG. 2, an analog timer circuit 200 in accordance withanother illustrative embodiment is shown. The analog timer circuit 200may be viewed as a particular implementation of the analog timer circuit100 previously described in conjunction with FIG. 1.

The analog timer circuit 200 comprises an astable multivibratorimplemented using amplifier U1A, resistors R1, R2, R3, R4, R5 and R15,capacitor C1 and diode D1. The multivibrator generates an asymmetricsquare wave of the type shown at the output of the amplifier U1A. Theasymmetric square wave is inverted by an inverter formed by amplifierU1B and resistors R6 and R7 and the resulting signal drives a capacitorcharge pump implemented by capacitor C2. The analog timer circuit 200operates from a supply voltage denoted +V.

The capacitor charge pump C2 is coupled through a charge distributioncircuit to a charge storage element implemented by storage capacitor C3.The charge distribution circuit comprises resistors R8 and R9 and diodeD2. Part of the positive output of capacitor charge pump C2 charges thestorage capacitor C3 through D2 and R9. The charge rate is controlled bythe ratio of the resistance values of resistors R8 and R9. The chargerate is also controlled by the ratio of the capacitance values of thecapacitors C2 and C3, and by the frequency and asymmetry of theasymmetric square wave generated by the multivibrator.

The output of the multivibrator is also coupled to the storage capacitorC3 via a discharge pump C4 and a discharge distribution circuitcomprising resistors R10 and R11 and diode D3.

The discharge rate is controlled by the ratio of the resistance valuesof resistors R10 and R11. The discharge rate is also controlled by theratio of the capacitance values of the capacitors C3 and C4, and by thefrequency and asymmetry of the asymmetric square wave generated by themultivibrator.

The analog timer circuit 200 further comprises an up/down driver circuitimplemented using amplifier U1C, resistors R12 and R13, and diodes D4and D5. The voltage at the upper terminal of the storage capacitor C3will either increase or decrease based on the output state of amplifierU1C. More particularly, the U1C output state will be either high or lowdepending on the state of the up/down input signal applied to thenon-inverting terminal of U1C via resistor R12. The inverting terminalof U1C receives a reference voltage established off the supply voltage+V by a divider comprising resistors R1 and R2. That reference voltageis also provided to the non-inverting terminal of U1B.

The up/down input signal illustratively takes on values of +V or groundpotential to put the analog timer circuit 100 into respective charge ordischarge modes of operation. A high output from U1C through diode D5will prevent C4 from discharging C3 in the charge mode of operation, anda low output from U1C through diode D4 will prevent C2 from charging C3in the discharge mode of operation.

The analog timer circuit 200 further includes an output driver. Theoutput driver comprises a source follower circuit coupled to aunity-gain isolation amplifier U1D. The source follower circuit isimplemented using a metal-oxide-semiconductor field effect transistor(MOSFET) Q1 and resistor R14. The gate of Q1 is coupled to upperterminal of storage capacitor C3. The drain of Q1 is coupled to thepositive supply voltage +V. The source follower circuit presents a veryhigh impedance to the storage capacitor C3. The unity-gain isolationamplifier U1D further provides a high impedance to the output of Q1 anda low impedance at its output. Considering the small energy pulsesdelivered by the charge and discharge pumps in this embodiment, theoutput of U1D can be considered to be substantially continuously varyingwith time.

As a more particular example, the analog timer circuit 200 can beconfigured utilizing the components and values listed below. Theseparticular components and values should not be viewed as limiting in anyway and can be varied in other embodiments.

U1A, B, C, D LM324 quad op-amp Q1 IRFF430 MOSFET C1 CK05BX104, 0.1 μfd,50VDC C2, C4 CK05BX103, 0.01 μfd, 100VDC C3 C430C225K5HA, 2.2 μfd, 50VDCD1-D5 1N4148 low-leakage signal diode R1, R2, R6, R12  10 KΩ R3, R13,R15 100 KΩ R4  2.2 MΩ R5 270 KΩ R7  22 KΩ R8, R11, R14  33 KΩ R9, R10470 KΩ

All of the resistors listed above are assumed to be 0.1 watt rated, 5%tolerance metal film resistors.

Using the particular components and values given above, the analog timercircuit 200 provides a charge time constant of about 2340 seconds. SinceR9 and C3 are 470 KΩ and 2.2 microfarads (pfd), respectively, thecorresponding RC time constant for these values is about 1.03 seconds.Accordingly, this example configuration provides a time constantmultiplication factor of about 2270:1.

An alternative configuration using values of 10 KΩ and 100 KΩ for R8 andR9, respectively, while keeping C3 at 2.2 μfd, provides a charge timeconstant of about 2160 seconds. The corresponding RC time constant forthese values is about 0.22 seconds. Accordingly, this alternativeconfiguration provides a time constant multiplication factor of 9818:1.

Further control of the charge time constant in the analog timer circuit200 can be provided by adjusting the value of capacitor C2 relative tothat of capacitor C3. More particularly, the charge time constant variesinversely with the value of the capacitor C2 for a given value ofcapacitor C3. Also, varying the frequency and/or asymmetry of the pulsesignal generated by the multivibrator can be used to control the chargetime constant.

Similar time constant adjustment mechanisms are provided for thedischarge time constant using the discharge portions of the analog timercircuit 200, including resistors R10 and R11 and capacitor C4.

Those skilled in the art will recognize that a wide variety of othertime constant multiplication factors can be provided in astraightforward manner for charge or discharge portions of the analogtimer circuit 200, as appropriate for a given application.

It should also be noted that the charge time constant multiplicationeffect need not be substantially the same as the discharge time constantmultiplication effect. The discharge time constant can be made differentfrom the charge time constant by, for example, configuring the analogtimer circuit 200 such that R8 is not equal to R11 and/or C2 is notequal to C4. Additionally or alternatively, different pulse signals withdifferent frequency and/or asymmetry characteristics from differentmultivibrators can be used for the respective charge and dischargeportions of the analog timer circuit.

It is to be appreciated that the analog timer circuit embodimentsdescribed above are presented by way of example only. Numerousalternative embodiments can be configured using additional oralternative components in other configurations. Accordingly, althoughillustrative embodiments of analog timer circuits have been describedwith reference to FIGS. 1 and 2, it is to be understood that analogtimer circuits in accordance with the invention are not limited to thoseprecise embodiments, and that various other changes and modificationsmay be effected by one skilled in the art without departing from thescope or spirit of the invention. For example, another illustrativeembodiment of an analog timer circuit will be described below inconjunction with FIG. 6.

Examples of feedback control systems incorporating an analog timercircuit in accordance with embodiments of the invention will now bedescribed in more detail with reference to FIGS. 3 and 4. These systemsare illustratively configured for controlling temperature of a heat/coldchamber and for controlling speed of a motor, respectively. Theheat/cold chamber and the motor are examples of what are more generallyreferred to herein as “controlled components.”

Referring first to FIG. 3, a feedback control system 300 comprisesanalog timer circuit 100 and a power control circuit 302. The analogtimer circuit 100 is assumed to be configured as an up/down timer, withcharge and discharge modes of operation, as previously described inconjunction with FIG. 1. The power control circuit 302 has an inputcoupled to an output of the analog timer circuit 100.

The controlled component in this embodiment comprises a heat/coldchamber 304. The heat/cold chamber 304 has an input coupled to an outputof the power control circuit 302. The heat/cold chamber 304 isillustratively configured for use in long-term temperature cycling ofelectronic components for reliability testing or normalization purposes,or other temperature cycling arrangements that utilize relatively longtime constants.

The feedback control system 300 further comprises a temperature sensor306 having an input coupled to an output of the heat/cold chamber 304and an output coupled to an input of the analog timer circuit 100 via anup/down select circuit 308. The temperature sensor 306 is an example ofwhat is more generally referred to herein as a “sensor circuit.”

The up/down select circuit 308 coupled between the temperature sensor306 and the analog timer circuit 100 is configured to control selectionbetween charge and discharge modes of operation of the analog timercircuit 100, responsive to an output signal generated by the temperaturesensor 306.

The FIG. 4 embodiment operates in a manner similar to the FIG. 3embodiment, but the controlled component in this embodiment is a motor.Such an arrangement can be utilized, for example, for long-term testingof the motor. A feedback control system 400 comprises analog timercircuit 100 configured as an up/down timer, a power control circuit 402,a controlled component comprising a motor 404, a sensor circuitcomprising a speed sensor 406, and an up/down select circuit 408. Theup/down select circuit 408 is configured to control selection betweencharge and discharge modes of operation of the analog timer circuit 100,responsive to an output signal generated by the speed sensor 406.

It is also to be appreciated that the particular control systemsillustrated in FIGS. 3 and 4 are exemplary only, and numerous othersystem arrangements incorporating an analog timer circuit may be used inother embodiments. For example, the disclosed techniques may be adaptedin a straightforward manner for providing analog timer circuits withrelatively long time constants in a wide variety of other systemapplications. Such applications can illustratively include any type ofsystem in which a substantially continuous slowly increasing ordecreasing analog timer output is needed.

As mentioned previously, an illustrative embodiment of hot water boilercontrol circuitry that utilizes an analog timer circuit will bedescribed below in conjunction with FIGS. 5, 6 and 7. Such hot waterboiler control circuitry in combination with other system componentsillustratively implements a feedback control system for control of a hotwater boiler in a residential or commercial environment. Numerous otherfeedback control systems can be configured in other embodiments. Forexample, control circuitry embodiments of the invention are not limitedto use in hot water boiler control applications.

Referring now to FIG. 5, boiler control circuitry in the form of boilercontrol 500 illustratively comprises current sense circuit 502, powersupply 504, reference voltages circuit 506, burner control circuit 510,burner/ignition driver 512, fault driver 514, dual boiler sense circuit516, fault detector 518 and alarm 520. The boiler control 500 furthercomprises an analog up/down timer circuit 200′ which illustrativelycomprises an analog timer circuit similar to the analog timer circuit200 previously described in conjunction with FIG. 2.

The boiler control 500 is assumed to be implemented in a heating systemcomprising a boiler, a burner configured to burn fuel to heat liquid inthe boiler, and one or more circulation loops through which heatedliquid from the boiler is circulated in order to heat a room or area,for example, in a residential or commercial building.

The heating system further comprises at least one circulator configuredto controllably circulate liquid from the boiler through the one or morecirculation loops and back to the boiler responsive to one or morecontrol signals.

The liquid heated by the boiler and circulated through the system insuch an arrangement is illustratively water, although other types ofliquid can be used, including, for example, water combined with varioustypes of coolants or antifreezes, as will be appreciated by thoseskilled in the art. Accordingly, references herein to hot water boilers,hot water boiler control circuitry and similar terminology referring towater or hot water should be construed as examples only.

The fuel that is burned to heat the liquid in the boiler may be, forexample, heating oil, natural gas, propane or other types of fuels. Inthe case of heating oil, ignition of a burner is typically associatedwith activation of a fuel pump to provide the heating oil to the burnerfor ignition. Similarly, in the case of natural gas, ignition of aburner is typically associated with activation of a gas valve to providethe natural gas to the burner for ignition. In some embodiments, acommon signal is utilized for both activation of the fuel pump andignition of the burner in the heating oil case, or for both activationof the gas valve and ignition of the burner in the natural gas case.References herein to terms such as “ignition control signal” and“ignition signal” are therefore intended to be broadly construed and maybe utilized in providing additional functionality other than ignitioncontrol or ignition, such as activation of a fuel pump or gas valve inconjunction with ignition.

The current sense circuit 502 is configured to detect activation of atleast one circulator arranged to circulate liquid from a boiler througha circulation loop and back to the boiler. More particularly, in thisembodiment, the current sense circuit 502 is arranged in series with analternating-current (AC) control line of the circulator. The currentsense circuit 502 is an example of what is more generally referred toherein as a “sensor circuit,” and other types of sensor circuits can beutilized to detect activation of one or more circulators in otherembodiments.

It is further assumed in this embodiment that the heating systemcomprises multiple circulators, all controlled by an AC control line,illustratively 115 volts AC. Activation of a given one of thecirculators illustratively refers to configuration of the circulator tocause liquid to be circulated through a corresponding circulation pathor loop. This can be viewed in some embodiments as turning thecirculator “on.”

The analog up/down timer circuit 200′ has an input coupled to an outputof the current sense circuit 502 and is configured to generate avariable threshold voltage VTadj that varies as a function of anactivation time of the circulator. The variable threshold voltage is anexample of what is more generally referred to herein as a “variablethreshold signal.” Other embodiments can utilize other types of variablethreshold signals, such as current signals or power signals, inimplementing a boiler control or other type of electronic controlcircuitry as disclosed herein, as will be readily apparent to thoseskilled in the art.

The burner control circuit 510 is configured to receive the variablethreshold voltage VTadj from the analog up/down timer circuit 200′ andto generate an ignition control signal based at least in part oncomparison of a temperature sensor voltage VT1 of the boiler with thevariable threshold voltage VTadj.

The burner/ignition driver 512 is configured to receive the ignitioncontrol signal from the burner control circuit 510 and to generate anignition signal for the burner based at least in part on the ignitioncontrol signal. As mentioned previously, such an ignition signal in someembodiments is assumed to be utilized not only for burner ignition, butalso to activate a fuel pump or gas valve in addition to igniting theburner in order to burn the fuel supplied thereto by the fuel pump orgas valve. This is the assumption in the present embodiment and thus theignition signal is more specifically denoted in the figure as aburner/ignition signal to emphasize that in this embodiment the ignitionsignal also controls fuel pump or gas valve activation. Numerous othertypes and arrangements of ignition signals and associated ignitioncontrol signals can be used in other embodiments. For example, in someembodiments, separate signals can be used for activation of a fuel pumpor gas valve and ignition of a burner to burn the fuel supplied by thefuel pump or gas valve.

In the present embodiment, the burner/ignition signal provided to theburner is more particularly generated by the burner/ignition driver 512operating in conjunction with the fault driver 514. The drivers 512 and514 may be collectively viewed as one possible implementation of an“ignition driver” as that term is broadly used herein.

The power supply 504 in this embodiment provides three distinct supplyvoltages, including supply voltages denoted V1, V2 and V3,illustratively 24 volts, 15 volts and 5.1 volts, all direct current (DC)voltage or vdc.

The reference voltages circuit 506 generates high and low temperaturelimit voltages denoted VThigh and VTlow, respectively, utilizing thesupply voltage V3. These high and low limit voltages are also referredto elsewhere herein as Vhigh and Vlow, and illustratively correspond tohigh and low temperature limits of 180° F. and 120° F., respectively.Other high and low temperature limits can be used in other embodiments.The high and low temperature limit voltages VThigh and VTlow are appliedas inputs to the analog up/down timer 200′ as shown, and utilized in thegeneration of the variable threshold voltage VTadj.

The dual boiler sense circuit 516 also receives the supply voltage V3,and generates first and second temperature sensor voltages VT1 and VT2from respective first and second temperature sensors that are arrangedto measure the temperature of the liquid in the boiler. The temperaturesensor voltages VT1 and VT2 are examples of what are more generallyreferred to herein as temperature sensor signals. The first temperaturesensor voltage VT1 is applied to the burner control circuit 510 forcomparison therein with the variable threshold voltage VTadj. The firstand second temperature sensor voltages VT1 and VT2 are applied as inputto the fault detector 518. The fault detector 518 also receives asinputs the high and low temperature limit voltages VThigh and VTlow.

The fault detector 518 is configured to monitor the first and secondtemperature sensor voltages VT1 and VT2 and to generate a faultindication signal if the first and second temperature sensor voltagesVT1 and VT2 differ from one another by more than a designated amount.The fault indication signal is applied to the fault driver 514 and isalso utilized to drive alarm 520, which may be configured to generate anaudible or visual alarm signal or combinations of multiple alarm signalsof different types. The fault detector 518 is an example of what is moregenerally referred to herein as a “fault detection circuit,” and othertypes of fault detection circuits can be used in other embodiments. Forexample, in some embodiments, the fault detector 518 can be configuredto detect conditions under which at least one of the temperature sensorvoltages VT1 and VT2 falls outside of a designated range. Numerousalternative arrangements are possible, as will be apparent to thoseskilled in the art.

The fault detector 518 further includes a reset input configured toreceive a reset signal, possibly from a push-button reset control switchaccessible to a user. This allows the fault detector 518 to be reset orcleared after detection of a fault. Other types of reset mechanisms canbe implemented in other embodiments.

Referring now to FIG. 6, the up/down timer circuit 200′ and the currentsense circuit 502 of the boiler control 500 are shown in greater detail.The up/down timer circuit 200′ is substantially the same as the analogtimer circuit 200 previously described in conjunction with FIG. 2, andincludes op-amps U1A, U1B, U1C and U1D, transistor Q1, capacitors C1-C4,diodes D1-D5, and resistors R1-R15, all configured as previouslydescribed. The exemplary component types and values for these circuitelements as listed above in the context of FIG. 2 can also be utilizedin an implementation of the FIG. 6 embodiment. Again, these particularcomponents and values should not be viewed as limiting in any way andcan be varied in other embodiments. The up/down timer circuit 200′ inthe FIG. 6 embodiment further comprises an output series resistor R16and an output voltage limiter comprising diodes D6 and D7. The diodes D6and D7 serve to limit the variable threshold voltage VTadj generated bythe up/down timer circuit 200′ to a range defined by the respective highand low limit voltages Vhigh and Vlow. The resistor R16 may have a valueof 1 KΩ, and the diodes D6 and D7 may comprise 1N4148 low-leakage signaldiodes. The up/down timer circuit 200′ in this embodiment operates fromthe power supply voltage V2 provided by power supply 504.

The current sense circuit 502 as illustrated in FIG. 6 comprises atransformer T1, diodes D8, D9, D10 and D11, a bipolar transistor Q2, acapacitor C5, and resistors R18, R19 and R20. The diodes D8-D11 areconfigured to form a full-wave bridge rectifier. The transformer T1 andfull-wave bridge rectifier D8-D11 are arranged to sense current in theAC control line of the circulators. A corresponding up/down controlsignal based on the sensed current is generated at the collector of Q2and applied to input resistor R12 at the up/down control input of theup/down timer circuit 200′. Suitable values for C5, R18, R19 and R20 inthis illustrative embodiment are 35 μfd, 0.2 Ω, 2.2 KΩ and 3.3 KΩ,respectively. The transformer T1 illustratively comprises an EE 24-25square stack core with a primary winding comprising 10 turns of #20 wireand a secondary winding comprising 300 turns of #34 wire. The diodesD8-D11 may comprise 1N4148 low-leakage signal diodes, and transistor Q1may comprise a 2N2222 transistor. As in previous embodiments, thesecomponent types and values are examples only, and can be varied in otherembodiments, as will be appreciated by those skilled in the art.

FIG. 7 shows additional portions of the boiler control 500 in greaterdetail. More particularly, this figure illustrates the power supply 504,the reference voltages circuit 506, the burner control circuit 510, theburner/ignition driver 512, the fault driver 514, the dual boiler sensecircuit 516 and the fault detector 518.

Some of the additional portions of the boiler control 500 as illustratedin FIG. 7 include connector designations TS1-1, TS1-2, TS1-3, TS2-1,TS2-2, TS3-1, TS3-2, TS4-1 and TS4-2. These designations illustrativelycorrespond to respective connectors, which can be altered or eliminatedin other embodiments.

The power supply 504 in this embodiment comprises a transformer T2,diodes D31, D32, D33 and D34, a capacitor C31, resistors R31 and R32,and Zener diodes Z1 and Z2. The diodes D31-D34 are configured to form afull-wave bridge rectifier. The transformer T2 is coupled to AC inputillustratively at 115 volts AC. The power supply voltages V1, V2 and V3are developed across the capacitor C31, Zener diode Z1 and Zener diodeZ2, respectively. The Zener diodes Z1 and Z2 illustratively have reversebreakdown voltages on the order of 15 volts and 5.1 volts, respectively.Suitable values for C31, R31 and R32 in this embodiment are 250 μfd,360Ω and 2.4 KΩ, respectively, although other values can be used inother embodiments. The transformer T2 can be in a standardconfiguration. For example, a commercially-available transformer such aspart number 3FS-424 from Tamura may be used.

The dual boiler sense circuit 516 comprises a pair of temperaturesensors illustratively in the form of respective thermistors RT1 andRT2, as well as additional resistors R33 and R34 and an adjustableresistor R35 configured to provide a balance adjustment mechanism forthe two temperature sensors. The resistance of the thermistors RT1 andRT2 decreases as the temperature of the liquid in the boiler increases.Accordingly, in this embodiment the thermistors RT1 and RT2 areimplemented as respective negative temperature coefficient (NTC)thermistors, although other types of temperature sensors can be used inother embodiments.

At a relatively low temperature of about 77° F., the resistance of RT1or RT2 is about 2057Ω, while at higher temperatures of 130° F. and 190°F. the resistance is about 700Ω and 257Ω, respectively. These latter twotemperatures in some embodiments may comprise respective low and hightemperature limits. The corresponding temperature sensor voltages aregiven by 0.8925 volts and 0.3685, respectively. Suitable values for R33and R34 are 3.3 KΩ, and a suitable nominal value for adjustable resistorR35 is 25Ω.

The burner control circuit 510 and the fault detector 518 areillustratively implemented utilizing op-amps U31A, U31B, U31C, U31D,U32A, U32B, U32C and U32D. The op-amps may collectively comprise a pairof LM324 quad op-amps. The op-amps U31A, U31B, U31C, U31D, U32A and U32Dare part of the fault detector 518, and the op-amps U32B and U32C arepart of the burner control circuit 510.

The temperature sensor voltages VT1 and VT2 from the dual boiler sensecircuit 516 are applied to non-inverting inputs of the op-amps U31A andU31B of the fault detector 518. The op-amps provide voltages Va, Vb andVc to inputs of op-amps U31C, U31D and U32D for use in fault detection.The fault detector further comprises resistors R36 and R37 associatedwith op-amp U31A, and resistors R38 and R39 associated with op-amp U31B.Suitable values for resistors R36, R37, R38 and R39 are 10 KΩ, 1 KΩ, 10KΩ and 2.2 KΩ, respectively. Additional resistors utilized to developthe voltages Va and Vc include R40, R41 and R42, which may be 4.7 KΩ, 1KΩ and 4.7 KΩ, respectively. Inputs to respective non-inverting andinverting inputs of op-amp U32D are provided from respective voltagedividers coupled to respective supply voltages V2 and V3. The voltagedividers comprise respective resistor pairs R54-R55 and R52-R53.Suitable values for R54 and R55 are 22 KΩ and 4.7 KΩ, respectively.Suitable values for R52 and R53 are 2.2 KΩ and 3.3 KΩ, respectively.

The op-amps U31C, U31D, U32A and U32D of the fault detector 518 haverespective feedback capacitors C32, C33, C34 and C35 arranged betweentheir respective outputs and their respective inverting inputs. Each ofthese capacitors illustratively has a value of 0.1 μfd. The outputs ofthe op-amps U31C, U31D, U32A and U32D drive respective light-emittingdiodes (LEDs) denoted LED 31, LED 32, LED 33 and LED 34, whichillustratively provide visual indications of respective detected faults.

The reference voltages circuit 506 receives the supply voltage V3 fromthe power supply 504 and utilizes resistors R43, R44, R45 and R56 togenerate the high and low temperature limit voltages Vhigh and Vlow.Resistors R43, R45 and R56 are illustratively 4.7 KΩ, 47Ω and 20.5 KΩ,respectively. Resistor R44 is illustratively an adjustable resistorhaving a nominal value of 1 KΩ, and provides a mechanism for adjustingVhigh.

In this embodiment, the fault detector 518 operates on both Vhigh andVlow in that Vhigh is developed from Vlow in the reference voltagescircuit 506. Other arrangements can be used to provide Vhigh and Vlow asinputs to the fault detector 518.

The burner control circuit 510 receives the temperature sensor voltageVT1 from the dual boiler sense circuit 516 at the non-inverting input ofop-amp U32C. The output of op-amp U32C is coupled to its inverting inputin a unity-gain configuration, and is also coupled via resistor R46 tothe non-inverting input of op-amp U32B. The inverting input of theop-amp U32B receives the variable threshold voltage VTadj from theanalog up/down timer circuit 200′. Op-amp U32B is configured with afeedback resistor R47 between its output and its non-inverting input,and with resistor R48 between its output and ground. Suitable values forR46, R47 and R48 are 10 KΩ, 1 MΩ and 10 KΩ, respectively.

The burner/ignition driver 512 in this embodiment comprises an npnbipolar transistor Q3 having its base coupled to the output of theop-amp U32B. The collector of Q3 is coupled via a coil of a relay switchcircuit K1 to the supply voltage V1. Protective diode D35 is arranged inparallel with the coil of the relay switch circuit K1. The relay switchcircuit K1 further comprises a switch arranged in series with an ACcontrol line of the burner, with the current through the coilcontrolling the state of the switch, based on the output of op-amp U32Band the corresponding on or off state of transistor Q3.

The fault driver 514 is configured in a manner similar to theburner/ignition driver 512. More particularly, the fault driver 514comprises an npn bipolar transistor Q4 having its base coupled to theoutput of the fault detector 518 via a resistor network comprising R49,R50 and R51, each illustratively 3.3 KΩ. The collector of Q4 is coupledvia a coil of a relay switch circuit K2 to the supply voltage V1.Protective diode D36 is arranged in parallel with the coil of the relayswitch circuit K2. The relay switch circuit K2 further comprises aswitch arranged in series with the AC control line of the burner, withthe current through the coil controlling the state of the switch, basedon the output of the fault detector 518 and the corresponding on or offstate of transistor Q4.

The first and second relay switch circuits K1 and K2 are arranged suchthat the burner will be ignited only if the current temperature sensorvoltage VT1 exceeds the variable threshold voltage VTadj and there areno detected faults. A fault is detected if any of the temperature sensorvoltages goes outside of its predetermined range. Such a detected faultwill advantageously prevent the burner from igniting, thereby preventingboiler temperature runaway conditions and potential catastrophicfailure.

Another advantage of the above-described embodiment of boiler control500 is that the circulators are controlled separately from the burnerignition. This is in contrast to conventional arrangements such as anAquastat that controls both the circulators and the burner ignition. Insuch a conventional arrangement, a failure of the Aquastat could preventboth circulator activation and burner ignition. With the boiler control500, the circulators could continue to run even in the presence of afault that prevents burner ignition. Such an arrangement could beparticularly advantageous in the event of a fault arising in freezingweather, as the water could continue to circulate through the system andtherefore be less likely to freeze, since moving water freezes at alower temperature than stagnant water.

The variable threshold voltage VTadj is increased or decreased by theup/down timer circuit 200′ based on activation time of the circulators,as detected by the current sense circuit 502. As mentioned previously,it may be assumed in some embodiments that a heating system comprisingthe boiler control 500 includes a plurality of circulators. In such anarrangement, if any one of the plurality of circulators is activated byapplication of a control signal to its control line, the current sensecircuit 502 causes the up/down timer circuit 200′ to gradually decreasethe adjustable threshold voltage VTadj. Thus, if only a single one ofthe multiple circulators is activated, or if two or more of the multiplecirculators are each activated, the adjustable threshold voltage VTadjwill be gradually decreased by the up/down timer circuit 200′. If all ofthe multiple circulators are deactivated, the current sense circuit 502causes the up/down timer circuit 200′ to gradually increase theadjustable threshold voltage VTadj.

Accordingly, in this embodiment, the boiler temperature will trackoutdoor temperatures based on on-off running times of the circulators.During typical winter months when the circulators are frequentlyactivated, the adjustable threshold voltage VTadj will be automaticallydecreased, while during typical spring, summer or fall months of lowerdemand for heat, the adjustable threshold voltage VTadj will beautomatically increased responsive to the reduced activation times ofthe circulators. The lowered boiler temperature when room or areathermostats are not calling for heat will advantageously reduce energylosses due to chimney convection, reduce fuel costs and effect areduction in greenhouse emissions. In addition, boiler system efficiencyis increased and furnace wear due to “short cycling” is eliminated.

It should be noted that the particular circuit elements and other boilercontrol components illustrated in FIGS. 5, 6 and 7 can be varied inother embodiments. For example, in other embodiments, the analog up/downtimer can be replaced with another type of timer circuit, possiblyincluding a digital timer. However, the use of an analog up/down timerprovides significant advantages in terms of reduced cost and complexity.

Additional illustrative embodiments will now be described with referenceto FIGS. 8 through 14. Each of these embodiments utilizes an auxiliaryheater to couple heat to a boiler temperature sensor responsive toactivation of the circulators, activation of the burner, or combinationsof both activation of the circulators and activation of the burner. Theauxiliary heater is also referred to as an auxiliary sensor heater. Inthese embodiments, the use of the auxiliary heater is based oncirculator or burner activation, which as previously described hereintends to track the outside temperature, in that activation times tend tobe longer for relatively cold outside temperatures than for relativelywarm outside temperatures. More particularly, the on time versus the offtime of the circulators and the burner tends to increase as outsidetemperatures decrease.

FIG. 8 shows a boiler control 800 that includes control circuitry 802comprising a furnace control circuit 804 and a low limit control circuit806. The term “furnace” in these embodiments is intended to represent anexample of a type of “boiler” as that term is more generally usedherein. The circuits 804 and 806 may comprise portions of a conventionalAquastat, although use of an Aquastat is not a requirement of any ofthese embodiments.

The furnace control circuit 804 controls activation of the furnace pumpand ignition of the burner responsive to a temperature sensor signalfrom a water temperature sensor 814. The low limit control circuit 806controls activation of an area circulator responsive to a control signalfrom an area thermostat 816. The low limit control 806 ensures that thecirculator is not activated until the boiler water is heated to the lowlimit temperature setting.

The water temperature sensor 814 receives boiler water heat denoted T1.The area thermostat 816 receives area heat T2 from the area in which itis installed. In this embodiment, the auxiliary heater is controlled byan auxiliary heater driver 820, which generates additional heat T3 whichis sensed by the water temperature sensor 814. The auxiliary heaterillustratively comprises an electrical heater that is coupled to thewater temperature sensor 814. More particularly, the electrical heatercomprising the auxiliary heater may be implemented as a fixed resistancepower resistor.

In other embodiments, other types of auxiliary heaters may be used,including by way of example auxiliary heaters based on positivetemperature coefficient (PTC) resistors. The use of PTC resistors canenhance performance in some embodiments by allowing application overwidely varying incoming voltage supplies. More particularly, theresistance of a PTC resistor can self-adjust for a given change in theapplied line voltage level in order to provide the proper auxiliary heatlevel.

The auxiliary heater is turned on by the auxiliary heater driver 820responsive to deactivation of the circulators, deactivation of theburner, or deactivation of both. This will reduce the boiler temperatureas the outside temperature increases. The lowered boiler temperaturewill reduce energy losses due to chimney convection, reduce fuel costsand finally effect a reduction in greenhouse emissions.

FIGS. 9, 10 and 11 are schematic diagrams of respective relay-basedimplementations of portions of the boiler control circuitry of FIG. 8.

Referring initially to FIG. 9, boiler control 900 comprises a burnerswitch S1, a circulation switch S2 and an area thermostat switch S3.These switches control activation of the burner and the circulators bycontrolling application of an AC control signal Vac to the burner viaswitch S1 and to the circulator via switches S2 and S3. Moreparticularly, the burner switch S1 controls activation of the burnerresponsive to the boiler water temperature, and the circulation switchS2 and area thermostat switch S3 collectively control activation of thearea circulator responsive to the boiler water temperature and the roomor area temperature, respectively. The boiler control 900 furthercomprises a relay Re1 that is operative to activate an auxiliary sensorheater R1 when the burner switch S1 is in its open position. When theburner switch S1 is closed, the auxiliary sensor heater R1 isdeactivated by the relay Re1. This arrangement effectively reduces thetrip temperature of the burner switch S1 as the outside temperatureincreases.

The boiler control 1000 of FIG. 10 is similar to boiler control 900 butthe relay Re1 is operative to activate the auxiliary sensor heater R1when at least one of the switches S2 and S3 is open. When both switchesS2 and S3 are closed, the auxiliary sensor heater R1 is deactivated bythe relay Re1.

The boiler control 1100 of FIG. 11 represents a combination of the FIG.9 and FIG. 10 embodiments. A first relay Re1 operates as in the boilercontrol 900 and a second relay Re2 operates as in the boiler control1000. Accordingly, in this embodiment, the auxiliary sensor heater R1 isactivated when switch S1 is open and when at least one of the switchesS2 and S3 is open. As a result, the auxiliary heat is coupled to theboiler water temperature sensor only when neither the burner nor thecirculator is activated. The auxiliary sensor heater R1 is deactivatedwhen S1 is closed or when both switches S2 and S3 are closed.

The electromechanical relays shown in FIGS. 9, 10 and 11 can be replacedby solid state circuitry. Examples of solid state circuit relayreplacements for FIGS. 9, 10 and 11 are shown in FIGS. 12, 13 and 14,respectively.

The boiler control 1200 of FIG. 12 represents a solid stateimplementation of the boiler control 900 of FIG. 9. In this embodiment,the AC signal Vac is rectified by D1 and the resulting DC voltage iscoupled to a resistor divider network comprising resistors R1, R2 andR3. The selected values of these resistors bias a power transistor Q1 toits on state. This in turn energizes an auxiliary heater comprising apower resistor R4 and provides the auxiliary heat to the boiler watertemperature sensor. When AC power is connected to the burner via theburner switch S1, it is rectified by diode D2 and the resulting DCvoltage is connected to the junction of R1 and R2. This back biasestransistor Q1 into its off state.

The boiler control 1300 of FIG. 13 represents a solid stateimplementation of the boiler control 1000 of FIG. 10. In thisembodiment, Q1 is back biased to its off state when the circulator isactivated via switches S2 and S3.

The boiler control 1400 of FIG. 14 represents a solid stateimplementation of the boiler control 1100 of FIG. 11. In thisembodiment, Q1 is back biased to its off state when either the burner isactivated via switch S1 or the circulator is activated via switches S2and S3. This embodiment utilizes three diodes D1, D2 and D3 arranged asshown, and Q1 will be biased off by voltage fed back by either diode D2or D3.

The illustrative embodiments of FIGS. 12-14 can be configured toautomatically adjust the effective resistance of the auxiliary heaterfor a change in the applied line voltage level in order to provide theproper auxiliary heat level.

It should again be emphasized that the above-described embodiments ofthe invention are presented for purposes of illustration only. Manyvariations and other alternative embodiments may be used. For example,the disclosed techniques are applicable to a wide variety of other typesof circuits and systems. Also, the particular configurations of circuitand system elements shown in FIGS. 1 through 14 can be varied in otherembodiments. Thus, for example, the particular types and arrangements ofcontrol circuitry elements and other components deployed in a givenembodiment and their respective configurations may be varied. Moreover,the various assumptions made above in the course of describing theillustrative embodiments should also be viewed as exemplary rather thanas requirements or limitations of the invention. Numerous otheralternative embodiments within the scope of the appended claims will bereadily apparent to those skilled in the art.

What is claimed is:
 1. An apparatus comprising: a sensor circuitconfigured to detect activation of at least one circulator arranged tocirculate liquid from a boiler through at least one circulation loop andback to the boiler; an analog up/down timer circuit having an inputcoupled to an output of the sensor circuit and configured to generate avariable threshold signal that varies as a function of an activationtime of said at least one circulator; a burner control circuitconfigured to receive the variable threshold signal from the analogup/down timer circuit and to generate an ignition control signal basedat least in part on comparison of a temperature sensor signal of theboiler with the variable threshold signal; and an ignition driverconfigured to receive the ignition control signal from the burnercontrol circuit and to generate an ignition signal for a burnerconfigured to burn fuel to heat the liquid in the boiler based at leastin part on the ignition control signal.
 2. The apparatus of claim 1wherein said at least one circulator comprises a plurality ofcirculators and the sensor circuit comprises a current sense circuitarranged in series with respective alternating-current control lines ofthe plurality of circulators.
 3. The apparatus of claim 1 wherein saidat least one circulator comprises a plurality of circulators and theanalog up/down timer circuit comprises: a pulse source; a charge storageelement; a charge pump coupled between the pulse source and the chargestorage element; and a discharge pump coupled between the pulse sourceand the charge storage element; wherein a pulse signal generated by thepulse source is utilized to charge the charge storage element via thecharge pump in a charge mode of operation for a period of time for whichat least one of the plurality of circulators is activated and todischarge the charge storage element via the discharge pump in adischarge mode of operation for a period of time for which none of theplurality of circulators is activated.
 4. The apparatus of claim 3wherein the analog up/down timer circuit is configured in a particularone of the charge and discharge modes of operation responsive to anup/down control signal generated by the sensor circuit.
 5. The apparatusof claim 3 wherein the charge pump and the discharge pump are driven byrespective ones of complemented and uncomplemented versions of the pulsesignal generated by the pulse source.
 6. The apparatus of claim 3wherein the analog up/down timer circuit further comprises: a chargedistribution circuit coupled between the charge pump and the chargestorage element; wherein the charge distribution circuit comprises firstand second circuit paths separating energy from the charge pump intorespective first and second portions; the first circuit path providingthe first portion of the charge pump energy to the charge storageelement; and the second circuit path diverting the second portion of thecharge pump energy away from the charge storage element.
 7. Theapparatus of claim 3 wherein the analog up/down timer circuit furthercomprises: a discharge distribution circuit coupled between thedischarge pump and the charge storage element; wherein the dischargedistribution circuit comprises first and second circuit paths separatingenergy from the discharge pump into respective first and secondportions; the first circuit path providing the first portion of thedischarge pump energy to the charge storage element; and the secondcircuit path diverting the second portion of the discharge pump energyaway from the charge storage element.
 8. The apparatus of claim 1further comprising a dual boiler sense circuit, the dual boiler sensecircuit comprising: a first temperature sensor configured to generatethe temperature sensor signal as a first temperature sensor voltage; anda second temperature sensor configured to generate a second temperaturesensor voltage.
 9. The apparatus of claim 8 further comprising a faultdetection circuit configured to monitor the first and second temperaturesensor voltages and to generate a fault indication signal if the firstand second temperature sensor voltages differ from one another by morethan a designated amount.
 10. The apparatus of claim 1 wherein theignition driver comprises a first relay circuit configured to control astate of a first switch arranged in series with an alternating-currentcontrol line of the burner based at least in part on the ignitionsignal.
 11. The apparatus of claim 10 further comprising a fault drivercomprising a second relay circuit configured to control a state of asecond switch arranged in series with the first switch and thealternating-current control line of the burner based at least in part ona fault indication signal generated by a fault detection circuit.
 12. Amethod comprising: detecting activation of at least one circulatorarranged to circulate liquid from a boiler through at least onecirculation loop and back to the boiler; generating a variable thresholdsignal that varies as a function of an activation time of said at leastone circulator; generating an ignition control signal based at least inpart on comparison of a temperature sensor signal of the boiler with thevariable threshold signal; and generating an ignition signal for aburner configured to burn fuel to heat the liquid in the boiler based atleast in part on the ignition control signal.
 13. The method of claim 12wherein said at least one circulator comprises a plurality ofcirculators and detecting activation of at least one circulatorcomprises sensing current in respective alternating-current controllines of the plurality of circulators.
 14. The method of claim 12wherein said at least one circulator comprises a plurality ofcirculators and generating a variable threshold signal that varies as afunction of an activation time of the circulator comprises: charging acharge storage element via a charge pump in a charge mode of operationfor a period of time for which at least one of the plurality ofcirculators is activated; and discharging the charge storage element viaa discharge pump in a discharge mode of operation for a period of timefor which none of the plurality of circulators is activated.
 15. Themethod of claim 12 further comprising: generating the temperature sensorsignal as a first temperature sensor voltage indicative of a temperatureof the liquid in the boiler; and generating a second temperature sensorvoltage also indicative of a temperature of the liquid in the boiler.16. The method of claim 15 further comprising: monitoring the first andsecond temperature sensor voltages; and generating a fault indicationsignal if the first and second temperature sensor voltages differ fromone another by more than a designated amount.
 17. The method of claim 12further comprising controlling a state of a first switch arranged inseries with an alternating-current control line of the burner based atleast in part on the ignition signal.
 18. The method of claim 17 furthercomprising controlling a state of a second switch arranged in serieswith the first switch and the alternating-current control line of theburner based at least in part on a fault indication signal.
 19. Aheating system comprising: a boiler; a burner configured to burn fuel toheat liquid in the boiler; at least one circulation loop; at least onecirculator configured to circulate liquid from the boiler through saidat least one circulation loop and back to the boiler; a sensor circuitconfigured to detect activation of said at least one circulator; ananalog up/down timer circuit having an input coupled to an output of thesensor circuit and configured to generate a variable threshold signalthat varies as a function of an activation time of said at least onecirculator; a burner control circuit configured to receive the variablethreshold signal from the analog up/down timer circuit and to generatean ignition control signal based at least in part on comparison of atemperature sensor signal of the boiler with the variable thresholdsignal; and an ignition driver configured to receive the ignitioncontrol signal from the burner control circuit and to generate anignition signal for the burner based at least in part on the ignitioncontrol signal.
 20. The system of claim 19 wherein said at least onecirculator comprises a plurality of circulators and the analog up/downtimer circuit comprises: a pulse source; a charge storage element; acharge pump coupled between the pulse source and the charge storageelement; and a discharge pump coupled between the pulse source and thecharge storage element; wherein a pulse signal generated by the pulsesource is utilized to charge the charge storage element via the chargepump in a charge mode of operation for a period of time for which atleast one of the plurality of circulators is activated and to dischargethe charge storage element via the discharge pump in a discharge mode ofoperation for a period of time for which none of the plurality ofcirculators is activated.